This will work. You'll want to remove all of the rust from the area, clean and prep the metal according to the instructions

This kit should satisfactory to complete that damage repair on your RV.

approximately 24" X 42"

The USC Fiber Glass repair kit(50766ZP) will work to repair a cracks in fiber glass vehicles and parts. Although it will bond to and seal the leak in your fiber glass pressure tank when properly prepaired, we can not state that it will support and hold the pressures you will be using. Not recommended for use in high pressure applications.

The resin will attack and dissolve the foam. The fiberglass component should be formed within a female mold of the part using gel coat to line the mold (for the finished, painted part surface) then fiberglass mat and or chopped strands saturated in resin and hand laying them in then pressing out al the air. The final part will be hollow. NOTE: This is advanced fiberglass work and should not be attempted without consulting a good book (many published) on constructing fiberglass parts.

Fiberglass cloth properly soaked in resin makes an excellent fiberglass structural repair. Carefully follow instructions to fully saturated the mat and expel any trapped air.

For this to work best you would want to encapsulate the entire wood panel in resin.

A fiberglass mat like this topped with body filler would give you a strong, smooth finish,